Add ir.message and use custom exceptions

issue3672

message.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0"?>
+<!-- This file is part of Tryton.  The COPYRIGHT file at the top level of
+this repository contains the full copyright notices and license terms. -->
+<tryton>
+    <data group="1">
+        <record model="ir.message" id="msg_user_email_unique">
+            <field name="text">E-mail of active web user must be unique.</field>
+        </record>
+        <record model="ir.message" id="msg_user_session_key_unique">
+            <field name="text">Web user session key must be unique.</field>
+        </record>
+    </data>
+</tryton>

tryton.cfg

@@ -5,3 +5,4 @@ depends:
     party
 xml:
     user.xml
+    message.xml

user.py

@@ -93,7 +93,7 @@ class User(DeactivableMixin, ModelSQL, ModelView):
             ('email_exclude',
                 Exclude(table, (table.email, Equal),
                     where=table.active == True),
-                'E-mail must be unique'),
+                'web_user.msg_user_email_unique'),
             ]
         cls._buttons.update({
                 'validate_email': {
@@ -414,7 +414,8 @@ class UserSession(ModelSQL):
         table = cls.__table__()
         cls.__rpc__ = {}
         cls._sql_constraints += [
-            ('key_unique', Unique(table, table.key), 'Key must be unique'),
+            ('key_unique', Unique(table, table.key),
+                'web_user.msg_user_session_key_unique'),
             ]
 
     @classmethod